The Field Effect Transistor (FET) has become the most common device used in integrated circuitry, providing a wide variety of electronic tasks, e.g., analog signal processing, memory functions, high speed, low power logic operations and power switching.
During the last decade the semiconductor industry has more fully embraced the use of protection circuitry to shield FET's and other circuitry from damage associated with brief, high power voltage spikes such as the ESD. Because the FET is in many instances the prevalent device type on such circuitry it is often most convenient to form transistor protection devices coincident with the fabrication of FETs. This allows economy, i.e., avoidance of extra fabrication steps.
Although formation of the protection device follows the process fabrication sequence of functional circuit FETs, the transistor operation which provides the over voltage protection is often based on bipolar action. That is, inherent in most FET structures there is a bipolar structure, sometimes termed a parasitic, which can be placed into conduction when certain minimum voltages are applied across input terminals of the integrated circuit.
In the past, parasitic devices associated with functional circuitry have sometimes provided unintended conduction paths that carry the ESD surge and create thermal damage along that path. As a solution, transistor devices are configured to form circuit paths which shunt the majority of damaging power to a ground terminal while avoiding more sensitive conduction paths through which transient, yet high current, high voltage conditions would cause damage.
Generally, efforts to economically incorporate ESD protection devices on an integrated circuit require compromises either in performance or manufacturing costs. As trends continue for increased circuit density and lower operating voltages it is becoming more of a challenge to avoid compromises between performance of functional circuitry and ESD circuitry. More specifically, these trends make it increasingly difficult to effectively shunt heat away from thermally sensitive regions. With decreased operating voltages, the provision of optimal protection should require that the ESD circuitry be tuned to more quickly respond to ESD events. While it is desirable to optimize the ESD device turn-on voltage in order to provide maximum protection before damaging the functional circuitry, it is recognized that when the parasitic devices are optimized to provide ESD protection, there can be less satisfactory performance of the functional circuitry.
CMOS integrated circuits with ESD protection transistors are shown in U. S. Pat. Nos. 5,559,352 and 6,444,511. Both patents show examples where the source and drain of the ESD device is provided with a p-implant beneath the source and drain. The p-implant is supposed to lower the breakdown voltage of the ESD device so that it triggers before the CMOS device fails. However, I found that the narrow p-implant tends to shift the ESD current laterally so that the ESD device directs the current beneath the gate and has too high a current density. That defect is caused in part by the making the p-implant late in the process where the contact opening is used as a mask for the P-type ions. The relatively narrow contact opening results in a narrow p-implant beneath the source and drain of the ESD device. This causes a high current density proximate the surface of the device, although it would be better if the current peaked deeper in the device.
Another trend which limits the performance of ESD protection circuitry relates to marked decreases in gate breakdown voltages. For device designs in the 0.25 micron regime it is common for gate thicknesses to be less than 60 Angstroms. To avoid damage to the FET gate insulator it is necessary to assure fast and satisfactory bipolar conduction during the transient event. It is also necessary to reduce the trigger-on voltage of the ESD device to values substantially lower than the gate breakdown voltage to conduct current along a path which avoids inflicting damage on the gate structure. Solutions which address this concern should have application in a wide variety of semiconductor products including those manufactured with CMOS, BiCMOS and power processes.